From EP A 1364574, it is known to mount side walls of a round baler movably relative to a frame, so that they can be moved farther apart or closer together. The movement is induced by vertically pivotable arms, which are raised to open the baling chamber. As the baling chamber is opened, the side walls are moved farther apart in order that the friction upon the end faces of the round bale decreases and the round bale can thus be more easily ejected.